Analysis method

ABSTRACT

A method is provided for determining the presence of compounds in aqueous solutions by utilizing repeated equilibrations of a nonreactive gas with an aqueous sample containing such compounds and analyzing the gas from such repeated equilibrations to determine compounds present in the liquid.

United States Patent 1 McAulitfe [111 3,759,086 1 Sept. 18,1973

[52] US. Cl. 73/19, 23/230 HC [51] Int. Cl B0ld 57/00 [58] Field ofSearch 73/19, 23, 23.1,

73/61.1; 23/230 R, 230 M,230 EP, 230 B, 230 HC; 55/38, 39, 53

3,060,723 10/1962 Kapff et a1 73/19 3,422,664 1/1969 Ayers 73/23.1

FOREIGN PATENTS OR APPLICATIONS 43/2163 12/1964 Japan 73/19 PrimaryExaminerRichard C. Queisser Assistant ExaminerAnth0ny V. CiarlanteA-ttorneyR. L. Freeland, G. F. Magdeburger, E. J. Keeling and J. A.Buchanan, Jr.

[57] ABSTRACT A method is provided for determining the presence ofcompounds in aqueous solutions by utilizing repeated equilibrations of anonreactive gas with an aqueous sample containing such compounds andanalyzing the {'56} Rehrences Cited gas from such repeatedequilibrations to determine UNITED STATES PATENTS compounds present inthe liquid. 2,861,450 11/1958 Ransley 73/19 2,987,912 6/ 1961 Jacobson73/19 4 Claims, 8 Drawing Figures EQUILIBRATION 2 ND. EQUILIBRATIONBENZENE IMPUIRITY CYCLOHEXANE EQUlLl-BRATION Patented Sept. 18, 1973 4Sheets-Sheet 4 N ZOCkKflT ZDUM oiom ONXON o xm INVENTOR CLAYTON D.McAl/L/FFE ANALYSIS METHOD BACKGROUND OF THE INVENTION This inventionrelates to a method of determining the presence of compounds in aqueoussolutions utilizing repeated equilibrations of a nonreactive gas withthe aqueous sample suspected of containing such compounds and analysisof the gas obtained from such equilibrations and, more particularly,this invention relates to measurements of widely varying concentrationsof hydrocarbons dissolved in water based on successive gaschromatographic analysis after repeated equilibrations of helium with anaqueous sample containing dissolved hydrocarbons.

Many fields require the measurement of widely varying concentrations ofhydrocarbons dissolved in water. These fields include pollution control,petroleum exporation, and biochemical research, among others. Severalgas chromatographic techniques are available for measuring hydrocarbonsin specific situations, but no one method satisfactorily meets therequirements for most analytical problems. Equilibration of the aqueoussample with an immiscible solvent has been used. However, the solventalso extracts nonhydrocarbon organic compounds, hydrocarbons are subjectto loss from the solvent, and the solvent often interferes with theanalysis. Direct injection of the water sample and elimination of thewater by adsorption, reaction, or retardation have also been used. Theamount of water that can. be handled by these methods limitssensitivity, and nonhydrocarbon organic compounds interfere.Exhaustively stripping the aqueous phase with a gas followed by trappingthe hydrocarbons in cold traps permits high sensitivity, butnon-hydrocarbon organic compounds again interfere. A procedure employinga single equilibration of a gas phase with the aqueous sample andanalysis of the gas phase gives high sensitivity and major separation ofhydrocarbons from nonhydrocarbon organic compounds dissolved in water.This method gives good results for the analysis of alkanes whichpartition to the extent of 96% or higher into a volume of gas equal-tothe volume of sample. However, cycloalkane, olefin, acetylene, andaromatic hydrocarbons partition from about -90% into the gas phase sothat calibrations must be made. Furthermore ionic solutes alter thepartition coefficient so that calibration with solutions of known ionicstrength is required.

BRIEF DESCRIPTION OF THE PRESENT INVENTION In a broad aspect the presentinvention provides for determining the presence of compounds in aqueoussolutions by isolating a volume of liquid containing such compounds insolution and adding to such isolated volume of liquid a known amount ofa gas which does not react with the compounds or the liquid. Equilibriumis established between the compounds in the solution and the nonreactivegas phase. The gas phase is separated so that only the liquid remainsand the gas is analyzed for compounds. The above steps are repeated onthe same liquid sample at least one additional time since it has beendemonstrated that the analysis of the gas phase after two successivephase equilibriums gives all data generally required in suchdeterminations.

In a more specific aspect, the present invention relates to multiplephase equilibrations for analysis of hydrocarbons dissolved in water.The present method retains the advantages of gas equilibration whilepermitting accurate hydrocarbon measurements on aqueous samples ofunknown ionic composition. It is based on successive gas chromatographicanalyses after repeated equilibrations of helium with an aqueous samplecontaining dissolved hydrocarbons. Alkane, cycloalkane, olefin,acetylene, cycloolefin, and aromatic hydrocarbons having up to 10 carbonatoms in the molecule can be determined. Gas chromatographic data on thesuccessive equilibrium gas phases are plotted and backextrapolated tothe hydrocarbon concentration in the original aqueous sample. Forhydrocarbons such as alkanes, which partition 96% or greater into thegas phase, it is more convenient and accurate to sum the amountextracted for the first two equilibrations instead ofback-extrapolating.

The method gives qualitative separation of hydrocarbons from highlywater-soluble organic compounds. Thus, dissolved hydrocarbons can bedetermined in various aqueous media, such as fresh water, seawater,subsurface brines, and biological fluids.

The method of the present invention is exemplified herein for theanalysis of hydrocarbons dissolved in water.'0ther embodiments of themethod permit many other diverse determinations. These includeestimation of distribution coefficients, Henrys law constants, vaporpressure, solubility and several related thermodynamic parameters. It isalso useful as a go/no-go test of high sensitivity.

PRINCIPAL OBJECT OF THE PRESENT INVENTION BRIEF DESCRIPTION OF THEDRAWINGS FIG. 1 is a graph showing results from three equilibrations forthree different classes of hydrocarbons in equal volumes of tap waterand helium at 25 C for successive equilibrations;

FIG. 2 is a graph illustrating partitioning of n-hexane between equalvolumes of tap water and helium at 25 C for successive equilibrations;

FIG. 3 is a graph illustrating partitioning of cyclohexane between equalvolumes of tap water and helium at 25 C for successive equilibrations;

FIG. 4 is a graph illustrating partitioning of toluene between equalvolumes of three salinity waters and helium at 25 C for'successiveequilibrations;

FIG. 5 is a graph illustrating determination of toluene in threesalinity waters and helium at 25 C for successive equilibrations;

FIG. 6 is a graph illustrating change in concentration of benzene-andcyclohexane in gas phase for successive equilibrations of helium withwater sample;

FIG. 7 is a graph illustrating four equilibrations showing organiccontaminants in tap water from a city water supply; and

FIG. 8 is a graph showing three equilibrations illustratin ghydrocarbons dissolved in tap water from a sample of crude oil withchromatograph attenuation constant.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides fordetermining the presence of compounds in aqueous solutions by firstisolating a sample of liquid containing foreign compounds in solution ina non-gaseous environment. A known amount of a gas which is not reactivewith the liquid or the foreign compounds is then added to the liquidsample in the isolated environment. Equilibrium is established betweenthe compounds in the liquid and the gas phase. The gas phase is thenremoved so that only the liquid phase sample remains in the isolatedenvironment. The gas phase is then analyzed for compounds and the abovesteps are repeated at least one more time on the isolated liquid sample.

In a preferred form the present invention provides for using ahypodermic syringe to make the multiple equilibrations. Typically, a50-ml glass hypodermic syringe with Luer-Lok fitting is flushed severaltimes with portions of the aqueous samples, of which 25 ml is finallyretained. 25 ml of helium, nitrogen or other hydrocarbon-freenonreactive gas is added and the syringe capped. It is then vigorouslyshaken (a paint shaker is recommended) for 3-5 min to establishequilibrium between phases. Twenty to 23 ml of the gas phase is thenflowed through the sample loop (preferably previously evacuated) of thegas chromatograph, and then a measured volume, usually from 1-10 ml ofthe gas is introduced for analysis.

The remaining gas in the syringe is carefully discharged by moving thesolution to the syringe tip, and 25 ml of fresh helium is added. Theequilibration process is repeated as many times as is required for thespecific application. If water should be lost during one of theanalyses, it is necessary only to add a correspondingly smaller amountof helium i.e., the ratio of the volume of gas phase to water phase mustbe kept constant. Similarly, temperature must be kept constant duringsuccessive equilibrations. During the equilibrations described hereintemperature was controlled to about :1" C by laboratoryair-conditioning. The materials, the chromatograph, the integrator, andcalibration procedures were described in Physical Chemistry 70, 1267(1966). This paper is incorporated herein by reference.

Table 1 set out below develops the significant mathematics. It isapparent that a plot of the log of the hydrocarbon concentration in thegas phase vs. the number of equilibrations produces a straight line.

The negative slope of this line is the log of the distributioncoefficient plus one. The intercept is the product of the initialconcentration of the unknown, this distribution coefficient, and aconstant related to the sample size, the instrument, and itssensitivity.

TABLE 1 MATHEMATICS OF MULTIPLE PHASE EQUILIBRATION Let X, quantity ofcompound 1 in the system during the ith equilibration G, quantity ofx inthe light (gas) phase, of volume V during the ith equilibration L,quantity of x in the liquid phase, of volume V during the ithequilibration Then Hx, ow /Liv, where Hx is the Henrys law constant ordistribution coefficient (2) But if V V and Hx, Hit a constant Then fromEquations 1 and 2 G I-IxX,/(Hx l) And the faction, f, of the total x ineach phase is f I-Ix/(Hx 1 fL 1 "f0 two adjacent equilibrations only.

Furthermore, generalization of Equation 8 gives log G,, an b where a=log (I-lx+l) b log I-IxXo Thus, a semilog plot of G, vs. n is linearwith the slope a function of Hx only and the intercept a function ofinitial sample composition, X0.

Although this may sound complex, the desired computations can readily beperformed. From the semilog line read any two adjacent gas phaseconcentrations.

Divide the greater by the lesser and subtract l. This is Hx, thedistribution coefficient. Note the intercept of the line, divide it byHx, and one has the desired concentration in the original sample aftercorrection for instrument response and samplesize. Data for thiscorrection are derived from analysis of gas samples of known compositionin the conventional way. These operations are exemplified in Table 2.

Variations in the way that different classes of hydrocarbons partitionis shown graphically in FIG. 1. The original solution was formulated intap water to give approximately the same peak heights for the firstequilibration for benzene, cyclohexane, and n-hexane. The results of thefirst equilibration shows approximately equal peak heights, as well as asmall peak for cyclopentane, which was an impurity in the cyclohexanestandard. The second equilibration resulted in a small n-hexane peak, asmall cyclohexane peak, and a toluene peak reduced by approximately Asmall benzene peak exists as an impurity from the toluene sample. Thethird equilibration shows no n-hexane peak, a very small cyclohexanepeak, a small benzene peak, and toluene as the major hydrocarbonremaining.

FIG. 1 graphically demonstrates that the first equilibration removesmost of the alkane, and that two equilibrations remove the majority ofcycloalkane, leaving predominantly aromatic hydrocarbon in the aqueousphase. Olefin and acetylene hydrocarbons would distribute between thewater and gas phases with different distribution coefficients fromalkane, cycloalkane, and aromatic hydrocarbons. If present in the watersample, they could also be identified and measured. For example, thepercentages of total hydrocarbon in the gas phase, calculated fromsolubilities and vapor pressures for equal volumes of gas and distilledwater for 1- pentene, 1,4-pentadiene, and l-pentyne are 94, 83, and 5l,respectively.

Because alkanes have low solubilities in water, 96+% partitions into thegas phase when equal volumes of gas and water are equilibrated. For'thisreason two, or at most three, equilibrations will transfer all of thealkanes present into the gas phase. FIG. 2 demonstrates this forn-hexane. The numerical values next to the points are the n-hexaneconcentrations in the gas phase for four separate analyses on tap water.Note that 99.8% of nhexane has been transferred after twoequilibrations.

The deviation of the third and fourth equilibration points from theline, magnified by the log scale and large slope, was caused byexperimental error. Each succeeding equilibration was slightlycontaminated by fluid from the preceding equilibrations, and was trappedbetween the plunger and the wall of the syringe during manipulation.

Because all 99%) alkanes are removed by two equilibrations, it isquicker and more accurate to sum the concentrations in the gas phases ofthe two equilibrations than to extrapolate to get the originalconcentration.

Cyclo alkanes partition less into the gas phase than do alkanes. FIG. 3shows the concentrations of cyclohexane in the gas phase for successive25 C equilibrations for three separate analyses of tap water containing55 ppm cyclohexane. Cyclohexane partitions 88.8% into the gas phase.After three equilibrations, cyclohexane has been 99.7% transferred, so asummation of the gas phase concentrations for three equilibrations givesthe concentration of cyclohexane in the water sample. A least squaresfit for three equilibrations gives a backextrapolated value of 54.8 ppm,a close check against 55.0 ppm, obtained by summation of the first threeequilibrations.

The results of the fourth and fifth equilibrations deviate from astraight line because of, as before, solution left between the plungerand cylinder of the glass syringe. After four equilibrations, one of thesamples was transferred to a clean dry syringe, and equilibrationsnumber five and six were carried out. These two points are in line withthe values at equilibration four, confirming that it was solutionexisting between the plunger and cylinder of the syringe that was thesource of error. This error, however, is of no great significance,because it does not become apparent until more than 99% of thehydrocarbon has entered the gas phase.

Because of their relatively high solubilities in water in relation totheir vapor pressures, aromatic hydrocarbons remain principally in thewater phase when equal volumes of water and gas are equilibrated. FIG. 4shows the actual data points for repeated equilibration of toluene withwaters of varying salinity. It is apparent that unless the water ishighly saline, many equilibrations would be required to extract themajority of the toluene. Therefore, the most convenient method fordetermining the original concentration is to fit a straight line to thedata points and back-extrapolate.

Although 10 equilibrations are shown for toluene, in

both tap water and 3.5% sodium chloride solution, two

or three equilibrations give reasonable accuracy and 4 or 5 as goodaccuracy as do 10 equilibrations.

One of the principal advantages of the present method is its ability todetermine hydrocarbon concentrations accurately in solutions of varyingionic composition. It matters little whether the water is fresh,brackish, seawater, or subsurface brine. FIG. 5, a plot of the datashown in FIG. 4, shows that within 1%, the same concentration isobtained from waters of widely varying salinity, even though thedistribution coefficients are markedly changed.'ln each case thesolutions were prepared to contain 42 ppm of toluene.

The analysis can be performed at different temperatures and willback-extrapolate as for varying salinity, provided temperatureconditions remain constant (11 C) during successive equilibrations.

The difference in distribution coefficients for different classes ofhydrocarbons can beused to determine hydrocarbons not resolved by thechromatographic column. As an example, on some columns cyclohexane andbenzene may not be completely resolved, as FIG. 6 illustrates. Throughrepeated equilibrations, separate, accurate analysis of benzene andcyclohexane can be made without attempting to resolve the overlappingpeaks.

From the chromatograms on FIG. 6 as well as the previous discussion ofcyclohexane analysis, it is apparent that over 99% of the cyclohexane isremoved from water after the third equilibration. Integration of theareas of the combined peaks for the first three equilibrations gives thetotal concentration of the cyclohexane plus a portion of the benzene.Back-extrapolation of equilibrations four through seven gives theconcentration of benzene in the original solution. The straight linethrough the data points for equilibrations four through seven alsopermits calculation of the benzene in the gas phase at equilibrationsone, two, and three. These benzene concentrations are subtracted fromthe areas measured for benzene plus cyclohexane. This gives then theconcentration of cyclohexane in the original solution.

The solution used in the analysis shown in FIG. 6 contained 70 ppmbenzene and 53 ppm cyclohexane in tap water. The described procedureproduced values of 69.6 and 53.1. An independent check was obtained byanalyzing a water sample containing benzene only. The value for benzenewas 70.2 ppm, showing that the cyclohexane did not alter the analysisfor benzene. Although the peaks for cyclohexane and benzene arepartially resolved, this procedure can be used even when peaks arecompletely superimposed.

The method of the invention gives good separation of hydrocarbons fromhighly water-soluble organic compounds, such as alcohols, aldehydes,ethers, and acids. Because of the high water solubilities of theseorganic compounds, the distributions are highly favored toward the waterphase, and little of them is found in the gas phase. If the water phaseshould contain a sufficiently high concentration of an organic compoundfor it to appear in the gas phase, then the gas phase concentrationdecreases very slowly with successive equilibrations. For example,diethyl etherpartitions -96% into the water phase when equal volumes ofwater and gas are equilibrated. This gives an immediate clue that thecompound is a nonhydrocarbon. Such information even in a qualitativesense can be quite useful.

Such distribution coefficients, along with relative retention time, helpidentify unknown organic compounds. An actual example of theidentification of organic contaminants in a tap-water sample from a citywater supply is shown in FIG. 7. The first equilibration shows thepresence of several organic compounds which, if this were the onlychromatogram, would be difiicult to identify. However, distributioncoefficients obtained by additional equilibrations permit positiveidentification of several of the contaminants.

The first peak is predominantly methane. The next prominent peak has therelative retention time for nhexane, but the additional equilibrationsgave a distribution coefficient that duplicates that for chloroform (ascalculated from solubility and vapor pressure data). Chloroform added towater gave the same results.

Benzene and toluene were identified also by relative retention times andpartitioning between gas and water phases. The peaks on either side ofbenzene have not been identified, but their distribution indicates thatthey are not hydrocarbons. Similarly, the peaks between methane andchloroform are not alkanes, which would have been completely removedafter two equilibrations; they may be olefins.

The concentrations in the tap-water sample are low (less than one ppb)but readily measurable. By contrast, only methane is detectable by thismethod in seawater samples from Cook Inlet, Alaska.

Although one would predict from FIG. 1 that a complex mixture ofhydrocarbons would partition as shown, it is always satisfying to seethe actual analysis of a complex mixture. Such a mixture, a sample ofcrude oil from the Black Hollow oil field near Ft. Collins, Colo., wascontacted with tap water. The light gases had been separated from thecrude oil, so C through C.,s were in low concentration.

The water containing the hydrocarbons dissolved from the crude oil wasanalyzed, and the results are shown in FIG. 8. The first equilibrationchromatogram shows all the hydrocarbons through eight carbon atomsnormally found in crude oils. As predicted, the first equilibrationremoved over 96% of the alkanes, leaving aromatic hydrocarbons and about10% of the cycloalkanes. The third equilibration chromatogram shows onlyaromatic hydrocarbons.

When we use the described procedure and introduce a 5-ml gas sample intothe chromatograph, the method is capable'of detecting alkane andcycloalkane hydrocarbons in water if they are present in amounts of oneto three parts in 10 parts of water by weight. Aromatic hydrocarbons,because of their lower partition ing into the gas phase, can be detectedif present in concentrations of 4-12 ppt. Reasonable accuracy can beobtained if the aqueous concentrations are 20 to 30 times these values.Methane is present in open ocean waters in amounts of 28-36 ppt. Withthe present procdure, methane can be detected at 1 ppt or less.Sensitivity can be increased by analyzing a larger sample of the gasphase and by increasing the ratio of water to gas.

Although the present invention has been described primarily inconnection with permitting accurate hyrocarbon measurements on aqueoussamples of unknown ionic compositions based on successive gaschromatographic analysis after repeated equilibrations of helium with anaqueous sample containing dissolved hydrocarbons, it is evident that themethod has applications of greater diversity. These applications includeestimation of distribution coefficients, I-Ienrys law constants, vaporpressure, solubility and several related thermodynamic parameters. Thusthe multiple equilibration method of the present invention has broadapplication. As noted, certain specific embodiments of the inventionhave been described. The invention is not to be limited to only suchembodiments, but rather to the scope of the appended claims.

I claim:

1. A method of determining the presence of volatile compounds insolutions comprising the steps of a. isolating in a non-gaseousenvironment a liquid sample containing foreign volatile compounds insolution;

b. adding to said sample a known amount of a nonreactive gas;

c. establishing equilibrium between compounds in solution and in gasphase;

d. separating said gas phase from said sample so that only liquid phasesample remains in a non-gaseous environment;

e. analyzing said gas phase for compounds;

f. repeating steps (b) through (e) at least one more time whilemaintaining a constant ratio of gas phase to liquid phase andmaintaining the temperature constant and g. determining the distributioncoefficient based on the concentration of compounds found in any twosuccessive steps (e).

2. A method of determining the presence of volatile compounds in aqueoussolutions comprising the steps of a. isolating in a non-gaseousenvironment an aqueous liquid sample containing foreign, volatilecompounds in solution;

b. adding to said sample a known amount of a nonreactive gas;

c. establish equilibrium between compounds in solution and in gas phase;

d. separating said gas phase from said sample so that only liquid phasesample remains in a non-gaseous environment;

e. analyzing said gas phase for compounds;

f. repeating steps (b) through (e) at least one more time whilemaintaining a constant ratio of gas phase to liquid phase andmaintaining the temperature constant and g. determining the distributioncoefficient based on the concentration of compounds found in any twosuccessive steps (e).

3. A method of determining the presence of volatile hydrocarbons inaqueous solutions comprising the steps of a. isolating in a non-gaseousenvironment a liquid sample containing volatile hydrocarbons insolution;

b. adding to said sample a known amount ofa nonreactive hydrocarbon-freegas;

c. establish equilibrium between hydrocarbons in solution and in gasphase; d. separating said gas phase from said sample so that only liquidphase sample remains in a non-gaseous g. determining the distributioncoefficient based on the concentration of hydrocarbons found in any twosuccessive steps (e).

4. A method of determining the presence of volatile hydrocarbons inaqueous solutions comprising the 5 steps of a. isolating in a hypodermicsyringe in nongaseous environment a liquid sample containing volatilehydrocarbons in solution;

b. adding to said sample in said syringe a known amount of a nonreactivehydrocarbon-free gas;

c. establish equilibrium in said syringe between hydrocarbons insolution and in gas phase;

d. removing said gas phase from said syringe while retaining said samplein said syringe so that only liquid phase sample remains in anon-gaseous environment in said sample;

e. analyzing said gas phase for hydrocarbons;

f. repeating steps (b) through (c) at least one more time whilemaintaining a constant ratio of gas phase to liquid phase andmaintaining the temperature constant and g. determining the distributioncoefficient based on the concentration of hydrocarbons found in any twosuccessive steps (e).

2. A method of determining the presence of volatile compounds in aqueoussolutions comprising the steps of a. isolating in a non-gaseousenvironment an aqueous liquid sample containing foreign volatilecompounds in solution; b. adding to said sample a known amount of anonreactive gas; c. establish equilibrium between compounds in solutionand in gas phase; d. separating said gas phase from said sample so thatonly liquid phase sample remains in a non-gaseous environment; e.analyzing said gas phase for compounds; f. repeating steps (b) through(e) at least one more time while maintaining a constant ratio of gasphase to liquid phase and maintaining the temperature constant and g.determining the distribution coefficient based on the concentration ofcompounds found in any two successive steps (e).
 3. A method ofdetermining the presence of volatile hydrocarbons in aqueous solutionscomprising the steps of a. isolating in a non-gaseous environment aliquid sample containing volatile hydrocarbons in sOlution; b. adding tosaid sample a known amount of a nonreactive hydrocarbon-free gas; c.establish equilibrium between hydrocarbons in solution and in gas phase;d. separating said gas phase from said sample so that only liquid phasesample remains in a non-gaseous environment; e. analyzing said gas phasefor hydrocarbons; f. repeating steps (b) through (e) at least one moretime while maintaining a constant ratio of gas phase to liquid phase andmaintaining the temperature constant and g. determining the distributioncoefficient based on the concentration of hydrocarbons found in any twosuccessive steps (e).
 4. A method of determining the presence ofvolatile hydrocarbons in aqueous solutions comprising the steps of ''a.isolating in a hypodermic syringe in non-gaseous environment a liquidsample containing volatile hydrocarbons in solution; b. adding to saidsample in said syringe a known amount of a nonreactive hydrocarbon-freegas; c. establish equilibrium in said syringe between hydrocarbons insolution and in gas phase; d. removing said gas phase from said syringewhile retaining said sample in said syringe so that only liquid phasesample remains in a non-gaseous environment in said sample; e. analyzingsaid gas phase for hydrocarbons; f. repeating steps (b) through (e) atleast one more time while maintaining a constant ratio of gas phase toliquid phase and maintaining the temperature constant and g. determiningthe distribution coefficient based on the concentration of hydrocarbonsfound in any two successive steps (e).